1. Field of the Invention
The present invention relates to a fiber scope which is to be inserted into dark and narrow gaps formed by a plurality of long articles as in an assembly of nuclear fuel rods in a nuclear reactor to observe the circumference thereof. In particular, the present invention relates to a fiber scope which is capable of observing a wide range while standing within point-blank range of an objective part thereof.
2.Description of the Prior Art
FIG. 1 is a sectional view showing an objective part of a conventional side-viewing type fiber scope. Referring to FIG. 1, a light which is transmitted by a light guide 11 is radially projected at an angle to the longitudinal axis of the light guide 11 by way of a prism (not shown) and the like which is provided at a pointed head of the light guide. The angle .alpha. of the light projection from the light guide 11 is limited by an outside diameter of a probe tube which is provided at the pointed head of the light guide 11 to generally be about 60.degree. at most. On the other hand, an image guide 12 is provided with a prism and a lens (not shown) at the pointed head thereof to catch a light from a range having a radial viewing angle .beta.. The viewing angle .beta. is likewise limited by an outside diameter of a probe tube provided at the pointed head of the image guide 12 and is generally limited to an angle of about 45.degree.. Since it is necessary that a reflected light from a range which is illuminated by a light projected by means of the light guide 11 be caught by way of the image guide 12, it is required to set the angles of the prisms of the light guide 11 and the image guide 12 so as to meet the requirement. However, when the gap between an object to be observed and a viewing part of the fiber scope is narrow, it is inconvenient that the light projecting range of the light guide 11 cannot entirely illuminate the field of view of the image guide even if the angle of the prism would be set an any angle. In other words, since .alpha., .beta. have the upper limit as described above, and since the light guide 11 is unable to be arranged excessively far away from the image guide 12 in respect of the limitation in uses, the following disadvantage appears:
In the case where a position which is spaced away from the image guide 12 by a distance H (for example about several hundreds mm) is to be observed, a field of view having a length D can be entirely illuminated due to the expansion of the light projecting range. However, in the case where the position which is spaced away from the image guide 12 by a distance h (for example several mm) which is sufficiently smaller than the distance H, merely a part (having a length of d') of the field of view having a length of d is illuminated. Accordingly, the observing range is narrow, and the inspecting efficiency is remarkably inferior.
Nuclear fuel rods in a nuclear reactor are positioned in a cooling pool in an arrangement having many narrow gaps thereamong with the rods arranged perpendicularly. Although nuclear fuel rods are generally constructed as a covering pipe containing nuclear fuel pellets, the covering pipe is periodically inspected to prevent radioactive fission products from escaping. A method of carrying out this observation in a simple manner has not been proposed. Although the conventional fiber scope has been applied to this observation, the gaps among nuclear fuel rods are narrow. Accordingly, the field of view effected by the projection from the light guide 11 is narrow and the inspecting efficiency is very low. As a result, the conventional fiber scope has never been practically used for this purpose.